As a packaging material, plastic film often requires good printability. The printability is determined by the wetting index. Plastic is well known to have a low wetting index. In particular, polypropylene has such a low wetting index that some surface processing, e. g., printing, cannot be achieved. In order to solve the problem and increase the wetting index, corona discharge treatment is generally used. However, the forcibly increased wetting index decreases in length of time.
Films made of conventional polypropylene resins such as propylene homopolymer or propylene-ethylene copolymer and -olefin copolymer have poor printability. Even when the wetting index is increased by corona treatment, it significantly decreases after being left for a long time at high temperature, so that the films cannot be used for printing. The surface tension of a biaxial oriented film and a non-oriented film made of homopolypropylene is in the range of 33-34 dynes/cm and increases to 40-45 dynes/cm after corona treatment for improving the printability. External environments such as heat, especially in the summer season, reduce the surface tension of the film so drastically in length of time that the workability such as printing and deposition deteriorates significantly.
In order to solve the problems, the techniques of blending the polypropylene resin with other resins having polar groups are being developed. However, blends are limited in compatibility and mechanical properties because of difference in basic backbone structures. Also, the graft polymerization techniques for introducing polar groups are being developed, but they are not commercially available and sufficient adhesion performance and printability are not achieved because of intrinsic complexity of the polymerization process and poor graft efficiency of polar monomers.